The present invention relates to a method and apparatus for preparing gypsum products (i.e., products comprising calcium sulfate dihydrate) from starting materials comprising calcined gypsum (i.e., calcium sulfate hemihydrate or anhydrite) and water. More particularly, the present invention relates to an improved apparatus for use in conjunction with the slurry mixer typically used in supplying agitated gypsum slurry to a wallboard production line. The present apparatus provides an improved mixing chamber which enhances the uniform mixing of foam into the gypsum slurry.
It is well known to produce gypsum products by uniformly dispersing calcined gypsum in water to form a slurry and then casting the slurry into a desired shaped mold or onto a surface and allowing the slurry to set to form hardened gypsum by reaction of the calcined gypsum (calcium sulfate hemihydrite or anhydrite) with the water to form hydrated gypsum (calcium sulfate dihydrate). It is also well known to produce a lightweight gypsum product by uniformly mixing an aqueous foam into the slurry to produce air bubbles. This will result in a uniform distribution of voids in the set gypsum product if the bubbles do not escape from the slurry before the hardened gypsum forms. The voids lower the density of the final product, which is often referred to as “foamed gypsum”.
Prior apparatus and methods for addressing some of the operational problems associated with the production of foamed gypsum are disclosed in commonly-assigned U.S. Pat. Nos. 5,638,635 and 5,643,510, which are incorporated by reference. The present invention relates generally to the use of foamed gypsum in the production of gypsum wallboard.
A gypsum wallboard mixer typically includes a housing defining a mixing chamber with inlets for receiving calcined gypsum and water, among other additives well known in the art. The mixer includes an impeller or other type of agitator for agitating the contents to be mixed into a mixture or slurry. Such mixers typically have a rectangular discharge gate or slot with a cutoff block or door. The discharge gate controls the flow of slurry from the mixer, and is difficult to adjust to change slurry flow when product requirements change, such as when thicker or thinner wallboard is desired.
Foam and/or other additives are generally added through a foam slot on the outer side of the cut-off block and about 2-4 inches before the gate's connection to a vertical canister and a donut or restrictor ring. In the production of gypsum wallboard, a balance must be maintained in that the foam is uniformly mixed into the slurry, but not broken down from excessive agitation. Within the canister, which is approximately 5-7 inches in diameter, the material spins downward. The canister is employed to reduce the flow pressure of the slurry discharged from the mixer, which typically operates in the range of 270-300 rpm. One drawback of such canisters is that a centrifugal force set up within the canister causes separation between the materials in the mix due to density differences.
More specifically, recent studies have shown that a vortex is created as the mixture flows in the canister, which also creates an empty air space in the canister. Such an air space is typically representative of unwanted buildup of material in the canister, which then more easily sets and causes clogging of the apparatus. Clogged mixing equipment causes costly downtime for repairs.
Another disadvantage of the vortex created in the canister is that higher density slurry components become separated from the relatively lower density foam. Due to centrifugal force, the slurry is pushed to the sides of the canister and the foam tends to stay in the middle. Thus, rather than providing a site for the uniform mixing of the foam and the slurry, the canister appears to be doing the opposite.
From the canister and donut ring, the material typically flows into a flexible, generally horizontal distribution boot, from where it is dispensed onto the wallboard paper web traveling with the flow of material. In some applications, the slurry is dispensed upon a previously deposited layer of relatively denser gypsum slurry. If the pressure of the dispensed slurry is too high, the previously deposited layer is disturbed, resulting in a condition known as “washout”.
Typically, a second facing is then applied on top of the slurry to constitute the second face of the gypsum board. Next, the sandwich passes through a forming station which determines the width and thickness of the gypsum board. This process is carried out in a substantially continuous operation, and the gypsum slurry begins to set immediately after the board is formed. Subsequently, the board is dried, cut and bundled into commercially acceptable lengths. Since the uniform mixing of foam within the slurry is a desired result in wallboard manufacture, in view of the drawbacks of the canister, the only places where the foam can mix with the slurry are the mixer gate and the flexible rubber boot.
Thus, it is an object of the present invention to provide an improved gypsum slurry mixing apparatus which does not require a canister.
Another object of the present invention of the present invention is to provide an improved gypsum slurry mixing apparatus and method which promotes uniform mixing of foam into the slurry.
Still another object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which causes a reduction in slurry flow pressure prior to discharge of the mixture upon the wallboard paper and at the same time promotes even mixing of foam into the slurry.
A further object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which reduces maintenance of mixing and dispensing equipment due to premature setting of the slurry.
A still further object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which increases slurry/foam mixing time while not requiring additional length of the board production line.
Yet another object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which provides an easily accessible mechanism for changing the volume of slurry emitted from the mixer.